1. Field of the Invention
The present invention relates to a pyrolytic boron nitride container. More particularly, the present invention relates to a pyrolytic boron nitride container suitable for retaining material melt used for growing a single-crystal of a III-V compound semiconductor by a Boat method, and further relates to a method of manufacturing such a pyrolytic boron nitride container.
2. Description of the Related Art
Methods of manufacturing semiconductor single-crystals of III-V compound semiconductors; e.g., GaAs single-crystals, GaP single-crystals, or InP single-crystals, are roughly divided into a Boat method and a Pulling method.
Of these methods, the Boat method is further divided into a horizontal Bridgman method (HB method), a vertical Bridgman method (VB method), a Horizontal Gradient Freeze method (HGF method), and a Vertical Gradient Freeze method (VGF method). According to these Boat methods, material melt is usually held in a container having a predetermined shape. While the material melt is being heated by a heat source provided outside the container, the heat source is moved, or a profile of heat is produced by the heat source itself, thereby producing a temperature gradient in the material melt. As a result, the material melt is solidified from the seed crystal-side so as to grow a single crystal.
In the Boat methods, a quartz boat is conventionally used as the container for retaining material melt. If quartz is used as material for the boat, Si, which is the constituent element of quartz, is mixed into a growing crystal as an impurity. Si acts as an amphoteric dopant with respect to III-V compound semiconductors. If single-crystals of a III-V compound semiconductor are grown by the Boat method through use of the quartz boat, there is usually employed a method in which a crystal is grown by doping it with Cr. However, the insulation performance of the crystal is reduced if the crystal is doped with Cr, and a resultant crystal becomes undesirable for use in IC substrates.
There arises a chemical reaction between the quartz boat and the material melt, and a so-called wetting phenomenon occurs, thereby making the crystal vulnerable to crystalline imperfections such as twinning ("Electronic Materials," Vol. 13, No. 1, pp. 32 to 37).
For these reasons, a pyrolytic boron nitride (PBN) container has recently come into use as a container for manufacturing undoped III-V compound semiconductor single-crystals having high purity. In the case where the PBN is used as a material for a boat, even when B and N which are the constituent elements of the boat are mixed in the single crystal as impurities, their levels are limited, so that they do not act as dopants. Consequently, electrical characteristics of the resultant III-V compound semiconductor single-crystals are not impaired.
The PBN has anisotropy in terms of heat conductivity in which the heat conductivity in the direction of the surface of stacked layers, namely, in the planar direction is 30 to 70 times that in thicknesswise direction. Therefore, the container acts so as to unify the temperature of material melt in both the circumferential and longitudinal directions, thereby hindering the production of a temperature gradient in the material melt according to the Boat method. This makes it difficult to control the temperature profile of the material melt within the container. As described above, the PBN has characteristics undesirable for a container used for manufacturing single-crystals by the Boat method. In practice, these characteristics of the PBN cause deterioration of the monocrystal rate of each grown crystal ingot, which indicates the rate of a monocrystal portion within each ingot.